how can l notice barrel wear by looking at the barrel? people say l have to looke at the throught of the barel. were is the throught? the end of the gun? l own a heym sr 30. 7mm rem mag, how many rounds will l have to fire through it so the gun wont be abel to hit a dear at 400 yards? thay say after 1000 - 1500 rounds efects acuracy, but thats probably just a slight amount. l wana know how many rounds will it take a 7mm rem mag to be useles at hunting at 400 yrds?